1. Field of the Invention
The present invention relates to barrier layer ceramic capacitors and more particularly to grain boundary barrier layer ceramic capacitors including a ceramic composition in which the surfaces of the individual grains of a ceramic semiconductor are insulated with nonconductive material which is diffused into the ceramic.
2. Description of the Prior Art
It is known in the art that barrier layer ceramic capacitors may be fabricated by forming an insulating layer on the surface grains of semiconducting ceramics such as barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3), and modified compositions of these titanates. These semiconducting ceramics are frequently doped with additions of various transition metals and rare earth elements to promote grain growth and to enhance the semiconductivity of the base ceramic material. Additionally, these dopants improve various electrical properties of capacitors so formed. Barrier layer ceramic capacitors are widely used because of their large apparent dielectric constant, small temperature coefficient of permittivity and small dielectric loss.
In order to produce barrier layer capacitors having large apparent dielectric constant (or permittivity) it is necessary to fire ceramic materials so that the semiconducting ceramic will have grain sizes ranging from 50-100 .mu.m. To obtain discs of semiconducting ceramic, a two step process is often used. In the first step, a semiconducting ceramic composition is sintered and subsequently converted to a semiconducting state by heating in a reducing atmosphere. Then either the surface of the semiconducting disc is reoxidized, or an insulating oxide is diffused into the disc. Electrodes are then formed on the resultant disc to produce a capacitor. Unfortunately, the final capacitor, while exhibiting a desirable large permittivity also exhibits a low cut-off frequency.
It is known from Maxwell-Wagner dispersion theory that a decrease in the grain interior resistivity will shift the capacitance dispersion in the direction of higher frequency. Because of the higher conductivity of semiconducting strontium titanate ceramics than semiconducting barium titanate ceramics, the cut-off frequency of the barrier layer capacitors based on strontium titanate is ten times higher than that of capacitors based on barium titanate.
The present invention overcomes many of the disadvantages of prior art devices such as poor high frequency performance by providing an improved barrier layer capacitor device which incorporates a novel semi-conducting ceramic material. The novel barrier layer ceramic capacitor according to the present invention is formed by a novel one-step process at a relatively low temperature which thereby produces an improved device at lower cost when compared to prior art devices.